Tesla coil, an electrical transformer that uses high-frequency alternating current (AC) to increase voltage. Because of its extremely high voltage, the electricity in a Tesla coil can travel through the air, powering—or damaging—nearby electronic devices, often with arcs of lightninglike electricity. Though the Tesla coil produces extremely high voltage, the high frequency of the current generally makes it possible for most people to approach the device and even be struck by the arcs without suffering injury. The spectacular effects created by the Tesla coil make the device popular for scientific exhibitions, but the principles underlying the coil were also important to the development of radio technology.
The Tesla coil was invented by Serbian American inventor Nikola Tesla in 1891. Tesla was primarily interested in its potential to wirelessly transmit electricity, particularly for lighting. He hoped to build large coils scattered across Earth, each of which would provide power to any device with a receiver coil. However, he had little success with this plan. In 1893 Tesla gave a lecture and demonstration on wireless transmission in which he proposed signal transmission in conjunction with power transmission. He also obtained a patent describing the same principles, which is considered the first radio patent.
A Tesla coil is an electrical transformer, or a device that raises or lowers voltage, which is a measure of electrical potential. A Tesla coil generates very high voltage, often in excess of one million volts. It uses AC electricity, meaning that the voltage of the circuit changes at a particular frequency. A modern Tesla coil usually consists of an initial transformer that boosts voltage from the power source and sends it to a capacitor attached to the primary coil, which absorbs the high-voltage power. When the capacitor reaches a sufficiently high voltage, electricity flows across a spark gap, or a space between two high-conducting terminals, at a high frequency, creating an AC current in the primary coil.
One of the key principles of the Tesla coil is resonance—achieving the frequency at which the device’s primary coil induces maximum voltage in the secondary coil. This is achieved through magnetic coupling, also called inductive coupling. The two coils are not tied together with a conductor; rather, electricity is run through the primary coil, which creates a magnetic field. This magnetic field creates an electrical current in the secondary coil, at a much higher voltage. To achieve this increase in voltage, the secondary coil must have many more windings than the primary coil. Frequently, the primary coil has only one or two windings, while the secondary may have hundreds or thousands. The secondary coil also contains a capacitor, which builds up extremely high voltage.
The resulting high voltage results in a magnetic field so strong that arcs of electricity flow like lightning from the Tesla coil to nearby objects. These lightninglike discharges can even flow to people, but this is usually dangerous only if a nearby person has a pacemaker or other medical device that could be affected by the Tesla coil. Though the voltages are very high, the impedance of the coil is high enough that only a small current is produced in humans who interact with it, and the frequency of the current is such that it has little interaction with nerve cells. A popular demonstration of a Tesla coil is to have a person hold a metal rod in one hand and a lightbulb in the other. Holding the rod close to the coil produces an arc of electricity that travels through the person’s body and lights the bulb. Fluorescent lightbulbs and other devices can also be powered by proximity to the Tesla coil, even without drawing an arc. It was this ability to produce wireless power that Tesla found so compelling.